A method has been developed that yields primarily extrachromosomal mutants of the fungus Neurospora crassa. This method will be utilized to produce a large number of mutants that will be classified by complementation analysis. In addition, mutants presently available will be utilized to study the assembly of cytochrome aa3. Chloramphenicol-treated cells will be utilized to study the assembly and regulation of the cyanide-insensitive alternate oxidase. The resp-2 mutant will be utilized to study the involvement of mitochondrial translation products in electron transfer complex I. A systematic study will be carried out on the mitochondrial ATPase of Neurospora. The enzyme will be purified and the genetics of assembly studied. An attempt will be made to select for extrachromosomal mutants at specific sites.